Electrochromism is a phenomenon where a material reversibly undergoes an oxidation-reduction reaction as a result of application of voltage and thus is reversibly changed in color. An electrochromic material exhibiting the electrochromism typically undergoes an oxidation-reduction reaction in a configuration where the electrochromic material is formed between two opposing electrodes and an ion-conductive electrolyte layer is filled between the electrodes. When a reduction reaction occurs adjacent to one of the two opposing electrodes, an oxidation reaction, which is a reverse reaction of the reduction reaction, occurs adjacent to the second electrode. That is, in a device using the electrochromic material, when voltage is applied, the electrochromic material is colored at both of the electrodes and is changed a hue or optical density.
There has been disclosed an electrochromic element including the electrochromic material (see, for example, PTL 1). When a transparent display device is produced using the electrochromic material, or when a device having a configuration, in which three coloring layers of cyan (C), magenta (M), and yellow (Y) are laminated, is produced for a full-color display, it is necessary that the electrochromic element is formed of a material that is colorless and transparent in a discolored state.
As the electrochromic material, there have been utilized viologen compounds exhibiting an electrochromic phenomenon where the compounds are colorless and transparent in a neutral state but are colored in a reduced state. When the viologen compounds are used, titanium oxide is suitably used. It has been reported that use of titanium oxide particles as electrochromic-compound bearing particles in a laminate is capable of achieving high optical density and a high contrast ratio.
There have been reported triarylamine compounds as an electrochromic material that is transparent in a neutral state but are colored in an oxidized state (see, for example, NPL 1). There has also proposed a combination of reactive, oppositely-polarized compound materials including a triarylamine polymer and a viologen compound (see, for example, PTL 2). However, the combination is unsatisfactory. This is because optical density and stability to repetitive use have been mentioned in the document, but coloring after repetitive use has not been studied.
Therefore, there has not been provided an electrochromic element that is operable at a relatively low drive voltage and is excellent in durability to repetitive use and responsivity.